The present invention relates to a gate turn-off thyristor and, more particularly, to a gate turn-off thyristor having independent transistor elements for controlling the turn-on and turn-off of the thyristor.
Heretofore, it is known to provide, in addition to a turn-on controlling transistor element, a transistor element for exclusively controlling the turn-off of high power gate turn-off thyristors (GTO thyristors). For example, according to V. A. K. Temple, "MOS CONTROLLED THYRISTORS (MCT'S)" IEEE IEDM Tech. Digest, pages 282-285 (1984), a GTO thyristor having an n-channel turn-on controlling FET and a p-channel turn-off controlling FET is disclosed in FIG. 4 of this reference. In this thyristor, the p-channel FET is switched to a conductive state in response to a drive voltage application to a gate electrode, and the n type emitter layer and the p type base layer of the GTO thyristor are electrically short-circuited therebetween, thereby controlling the turn-off of the thyristor.
According to the GTO thyristor, it is, however, difficult to effectively control the turn-off operation. This is because the turn-off current hardly flows due to a lateral resistance inherent in a p type base layer in the region of the p type base layer (e.g., the central portion of the GTO thyristor) relatively remote from a channel region (off-channel) where the turn-off current initially starts flowing, resulting that the turn-off is delayed in this region. More specifically, according to this example, the surface portion of the p type base layer, which becomes conducted when the GTO thyristor is turned off with the p type region (formed in the n type emitter layer) of high impurity concentration via the off-channel, functions also as a turn-on channel. In other words, the surface portion of the p type base layer of low impurity concentration is used directly as the on-channel of the turn-on controlling FET. Therefore, it is not allowed to alter the resistivity of the turn-on channel to a low value. Because, if the impurity concentration of the surface portion of the p type base layer is high enough to reduce the resistivity of the p type base layer, the threshold voltage level of the turn-on channel increases, thereby making it impossible to drive the turn-on of the thyristor effectively.
Further, according to the conventional GTO thyristor with the turn-on and turn-off controlling FETs, its withstand voltage cannot be enhanced as required. In the thyristor, it is necessary to set the resistivity of the n type base layer to a relatively high value (i.e., low impurity concentration) in order to enhance the withstand voltage of the thyristor. However, if this is so, the resistivity for the turn-off current will inevitably increase, thereby reducing the current capacity at the turn-on time.